The lid loosens up can't keep screws tight.
Teflon tape might work. Wrap it around your screw and try again. Each roll of tape from any hardware store is about 50¢ so it's not a huge investment! If that fails.. you might try a drop of clear glue on the plastic stem part of your screw.. not on the porcelain. Good luck!!
Replace the nuts and bolts. They are made of nylon and the threads do get stripped/worn.